The use of electromagnetic meters to measure the flow of blood or other liquids is well known. The basic concept, for example, is described in U.S. Pat. No. 2,149,847 which patent is hereby incorporated by reference. By passing blood, in a tube or other blood vessel oriented at right angles to a magnetic field, an electromagnetic force is induced in a direction mutually perpendicular to the magnetic field and the blood flow, since the blood has the property of a moving conductor cutting through a magnetic field. The voltage induced is proportional to the velocity of flow and therefore directly proportional to the volume rate of flow of the fluid. The voltage can be measured by means of electrodes positioned at diametrically opposite points of the tube along a diameter extending perpendicular to the magnetic lines of flux.
Electromagnetic flow meters of the type just described are disclosed in U.S. Pat. No. 4,195,515 to Smoll, U.S. Pat. No. 4,346,604 to Snook et al. Another example is disclosed in U.S. Pat. No. 4,881,413 to Georgi et al. entitled Blood Flow Detection Device which is hereby incorporated herein by reference in its entirety.
An important feature of all of the foregoing flow meters involves the positioning of the electrodes, the configuration of the magnetic structure, and the blood flow at a precisely fixed predetermined relationship in order to maintain an accurate calibration of the meter. In the past, it was common for the assembled tubes, electrodes, and magnetic structure to be constructed as a unitary assembly to insure proper measurement of blood flow. The unitary structure was found to have several disadvantages in the practice of medicine; in particular, such unitary construction presented problems with sterilization of the flow measuring device and with limitations on blood flow-rate sensitivity capabilities.
The above-referenced Georgi et al. '413 patent proposed an improvement to the electromagnetic flow meters wherein the conduit portion of the meter that comes directly in contact with the patient's blood was made separable from the rest of the flow detection unit. This allowed the conduit portion to be disposed of after use, or at least for the conduit portion to be separately sterilizable. However, the sliding tray arrangement used in the Georgi et al. device to support the removable flow conduit during operation of the device was difficult to manipulate and in some cases did not sufficiently secure and prevent movement of the conduit. The instability of the conduit with respect to the magnetic field generated by the device can lead to problems with zero drift stability and calibration.
The Georgi et al. '413 patent also proposed the utilization of a field focusing device permitting precise control of the electromagnetic field through which the blood flows, in order to enhance the sensitivity of the meter and minimize susceptibility to "noise" inherent in the detection system.
Notwithstanding the various advances in the technology that have been made over the years, it is the inventors' belief that there continues to be room for improvement of electromagnetic flow meters. In particular, the size and strength of the magnetic field required and the relatively low input impedance of the amplifier receiving the induced-voltage signals tend to lead to problems with zero drift stability. Also, noise susceptibility, differential rejection by the amplifier, and stray capacitance are a problem due to the placement of the input amplifier remote from the meter.
In flow meters of the type under consideration herein, the velocity of flow is sensed by the voltage generated by the moving conductive fluid in a magnetic field. To get an accurate interpretation of the total cross-section of the flow channel or flow conduit, the field must be as uniform as possible and extend along the channel for at least a distance equal to the diameter of the channel. The field must maintain a stable distribution in the sensing area when conductive or ferromagnetic material is moved around the outside of the sensor assembly. In prior art "unitary" construction flow meters, precise configuration of the magnetic field was not as difficult as with more recent flow meters having removable flow conduit sections. This is because the structure for establishing the magnetic field must be arranged to allow for insertion and removal of the disposable flow conduit. Typically, this involves providing a two-piece magnetic core structure, with one part being moveable with respect to the other. The gap between the two pieces is difficult to precisely control.
The intended application of this flow meter is to monitor the flow of blood to a patient that is undergoing open heart surgery. The flow of oxygenated blood being returned to the patient passes through the flow meter. The meter itself is part of the pump control console and has its output signal displayed to the operator of the pump so flow rate adjustments can be made as needed to maintain the patient's circulatory system.
Patient safety is the overriding concern in the design. The electric shock hazard is minimized by moving the primary isolation barrier into the flow meter itself, rather than at the signal processing circuit as is done in other blood flow meters.